Flickering lights are almost certainly the result of a power interruption. If it happens frequently, I would suggest you try to keep a log and talk to your power supply company. The flickering will reduce the life of lamps and other equipment.
As for your ADSL, yes the modem will loose power for a few 10s of milliseconds, drop sync, re-establish, drop, and so on.
Several of those and the DLM will note several losses of sync and thus reduce the speed to stop it occurring again. A small UPS will sort it, and if you use that as a supply for your PC too, that will continue working.
M H C
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